The Food and Drug Administration said Tuesday it is conducting a safety review of certain types of birth-control pills to see if they increase the risk of blood clots beyond that of other pills.

The agency said the review is focusing on products that contain drospirenone, which is used in pills including Yaz and Yasmin, which are marketed Bayer AG. Drospirenone is a type of female sex hormone called a progestin.

Most birth-control pills contain two types of hormones, estrogen and progestin. All types of pills increase the risk of blood clots. However, the FDA said two recently published studies reported a greater risk of blood clots for women taking birth-control pills containing drospirenone, compared to birth-control pills containing another progestin known as levonorgestrel.

The FDA said some previous studies have reported that the risk of blood clots for women who use birth-control pills containing drospirenone is higher than that for women who use birth-control pills containing levonorgestrel, while other studies haven't found such a risk. While the risk of blood clots is low among women who take birth-control pills, the FDA said it is higher than the risk among women who aren't taking the pills.

Blood clots form inside a vein and are known as deep vein thrombosis, or DVT. The clots usually form in the lower leg or thigh, but can break loose and travel to other areas of the body such as the lungs, where they are called a pulmonary embolism, or PE. The FDA said the symptoms of a DVT include the new onset of persistent leg pain, while those of a PE include severe chest pain and sudden shortness of breath.

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